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It is good practice to reduce the pressure in a pipeline prior to inspecting damage. One of the purposes of this pressure reduction is to prevent a ‘time dependent failure’ whilst inspecting the damage. The EPRG (European Pipeline Research Group) guidelines for the assessment of mechanical damage recommend that the internal pressure be reduced to 85 percent of the pressure at the time of damage to prevent time dependent failures. The PRCI (Pipeline Research Council International) Pipeline Repair Manual recommends a pressure reduction to 80 percent of the pressure at the time of damage. Failures that occur under a constant load are time dependent failures. This means that a defect in a pipeline could fail sometime after the damage was caused, even though there has been no increase in the applied load, or an active growth mechanism such as corrosion or fatigue. They have been observed during hydrostatic test hold periods, during operation and under laboratory conditions. Failures under constant load occur because plastic deformation occurs in the material surrounding a defect subject to a load, and because plasticity is time dependent. Time dependent behaviour is relevant to: (1) the safe working practices in the vicinity of a damaged pipeline, (2) pressure reversals, (3) the margin between the operating pressure and the hydrostatic test pressure, and (4) the minimum duration of a hydrotest. The results of experimental and analytical studies of time dependent behaviour reported in the published literature are reviewed in this paper, to understand the background to the above recommended pressure reductions. This paper is based on the findings of the Pipeline Defect Assessment Manual (PDAM) project, a Joint Industry Project sponsored by sixteen international oil and gas companies